Field of the Invention
One or more embodiments of the invention are related to the field of radio-frequency identification (RFID). More particularly, but not by way of limitation, one or more embodiments of the invention enable a passive RFID tag that provides long-range reception and that is insensitive to the surface on which it is mounted.
Description of the Related Art
RFID systems use a specialized interrogator which transmits an RF (radio-frequency) signal and detects the signal reflected by a distant target, known as a “tag”. The passive RFID tag has no battery and no transmitter. The tag has an antenna and a specialized RFID chip. The antenna intercepts a portion of the transmitted energy and delivers it to the chip. In this way, it is similar to the antenna on a conventional radio receiver. The principal difference is that the intercepted RF energy is rectified and used to power the RFID chip. Initially, and prior to powering up the circuitry in the chip, the input impedance to the chip is high. The antenna is resonant at the design frequency of operation, and therefore will act as a scatterer: it will absorb and reradiate a portion of the energy. The energy reflected by the tag may be received by the interrogator.
Initially, the interrogator may transmit an unmodulated carrier, providing power to the RFID chip. Subsequently, the interrogator may modulate the signal, transmitting a code to the RFID tag which may trigger some operation in the chip.
The purpose of the RFID chip is to modulate the signal from the interrogator by alternately lowering the impedance of the antenna terminals, i.e. shorting them, and returning them to the high-impedance state, i.e. opening them. This will cause the reflection to be strong in the high-impedance state, and weak in the low-impedance state. This enables the RFID chip to transmit information back to the interrogator without the need for a local power source or a conventional transmitter. This allows the RFID tag to be inexpensive and therefore disposable after its operational life has ended.
The RFID tag, therefore, has three main functions: (1) Intercept as much RF energy as possible for its physical size (have a large effective aperture), and re-radiate it, thus having a large radar cross-section (RCS) (i.e. a scatterer). (2) Deliver intercepted energy to an RFID chip by effective impedance matching. (3) Under control of the RFID chip, change the tuning of the antenna so as to selectively be a poor scatterer having a small radar cross-section (RCS), thus delivering encoded information in the reflected energy.
The RFID interrogator, or reader, is a conventional transmitter of a certain power level, and uses an antenna which may be directional. The product of the transmitter power and the antenna gain (inversely related to its beamwidth) determines at what distance the RF field strength is sufficient to power the RFID chip for a given tag configuration. The efficiency and gain of the RFID tag antenna affects the threshold strength of the RF field required to operate the RFID chip.
RFID tags are useful for detecting and tracking objects in various environments. The RFID chip may also be capable of storing and retrieving data specific to the object it is attached to. It is understandable how these low-cost tags are useful for tagging boxes and bins in a warehouse environment. Boxes and bins on a conveyor belt, for example, may be identified via their tag as they pass a specific physical point along the belt. Also, the tags may have information written to them as they enter or leave areas of the warehouse.
Many boxes or, for example, plastic bins have ample non-conductive space surrounding the side of the container to allow an RFID tag to be affixed without appreciably affecting its RF behavior. In the event of a cardboard box, for example, the tag can operate effectively during the useful life of the box, and be discarded with the box due to its low cost. At the end of the useful lifetime, the tag may be rendered inoperable by the interrogator so as not to interfere with the reading of other, useful tags.
Some applications require a tag to be affixed to a surface which is conductive. In this case, the design of the RFID tag antenna and its housing must take the conductive surface into account. Tags that are designed, for example, to be affixed to cardboard boxes may effectively cease to operate when affixed to a conductive surface.
Examples of conductive surfaces which may require a passive RFID tag include, for example, file cabinets, laboratory containers, computer components (servers, disk drives), and disposable foil containers for food or industrial ingredients.
Another important application for RFID tags is identifying and tracking people. People-tagging applications may include control of access to secured areas, tracking and timing of people crossing the finish line in a race, tracking prisoners in prison environments, tracking and identifying patients in a hospital situation, and tracking participants in an amusement or sports venue. There are several issues when tagging people including conductivity of the human body, RF blockage from the human body and methods of attaching the tag such that it is acceptable to the person while remaining effective. These issues may reduce the range over which the tag may be interrogated when compared the more benign environs of a cardboard box.
In order to be useful in people-tracking and -tagging, a passive RFID tag must therefore be very efficient and relatively insensitive to the surface it is mounted on. Further, it must be small enough such that a person will be able to support it on their body without significantly affecting the ability to perform chosen tasks. It must be capable of being interrogated over useful distances depending upon the application. Ideally, it must be of sufficiently low cost such that it may be discarded when its useful life has been exhausted, or may be given to a person to keep for long term use without great expense or the need for continued maintenance.
In all applications, it is generally desired for the tag to be physically as small as possible. Since the RFID chip itself is very small, the limiting factor for the total physical size is generally the antenna. As in all antenna applications, there are tradeoffs which put a lower limit on the achievable antenna size. In general, when relative antenna size is discussed, it refers to the ratio of the physical size with the size of a wavelength at the subject frequency, in free space. The physical size for the purpose of discussing efficiency is considered a sphere with the smallest radius that would enclose the antenna.
There exists a three-way tradeoff between antenna size, bandwidth and efficiency. Thus, the cost of size reduction is some combination of a reduced bandwidth and loss of efficiency. In small antennas, bandwidth is generally defined by the impedance matching, for example where the return loss is greater than 10 dB, or VSWR is less than 2:1.
Bandwidth reduction of a size-reduced antenna has performance implications in practice. If the resultant bandwidth is too narrow, it may not have acceptable performance over the desired band of operation. For example, in the USA RFID transponders may operate in the ISM band 902-928 MHz, a fractional bandwidth of about twenty-nine percent (29%). This is a fairly wide bandwidth for any size-reduced antenna. For example, the bandwidth of a thin dipole antenna is on the order of ten percent (10%). Therefore, bandwidth of the antenna is a relevant limiting factor in tag performance.
Efficiency is the ratio of achieved performance to theoretically ideal performance. It is manifested as signal loss in the antenna due to losses in the antenna structure, or losses coupled into the antenna circuit from adjacent materials. When the antenna is in the vicinity of a good conductor, it may detune the antenna, but losses are generally not coupled in. When the conductor has significant resistance, the associated RF losses will appear in the antenna circuit by coupling. The human body, for example, is a lossy conductor. Antennas in the vicinity of the human body will generally have reduced efficiency. The choice of antenna type will greatly affect this impact on efficiency.
As antennas are made physically small, they generally take one of two forms: small dipole or small loop. A small dipole is like a full-size dipole but it is generally shortened through the use of inductive loading. This can be in the form of a meander line or lumped inductors. When this happens, its input impedance goes up in magnitude. The near-field of such an antenna is dominated by the electric field. A small loop antenna has the opposite situation. As a loop antenna gets smaller, its near field is dominated by the magnetic field. A loop antenna generally uses a capacitance between the ends of the loop to resonate the loop. As a loop gets smaller, its input impedance gets smaller in magnitude.
In the vicinity of the human body, antennas with a dominant electric field such as small dipoles, are affected more than those that have a dominant magnetic field. This is because the human body has a very high relative dielectric constant (or permittivity) and a low relative permeability. Therefore a small loop antenna is a more desirable antenna when used near the human body than a small dipole antenna.
Most RFID tags, and much of the prior art, take the form of shortened dipoles, especially those that are designed into labels or “inlays” for application to cardboard boxes or similar containers. Other types of antennas intended for use in the UHF frequency band include ¼ wave antennas that require a dielectric in the middle, and that may not use a resonating capacitor. These types of antenna are by definition larger, being ¼ wavelength structures, than the apparatus detailed herein and are an order of magnitude more expensive. Known antennas of this type do not maximize the electric field on top, but rather on the edges and thus inherently have high loss. Therefore, tags based on either shortened dipole antennas or ¼ wave antennas are poor choices for applications on the human body or other conductive surfaces.
For at least the limitations described above there is a need for a long-range surface-insensitive passive RFID tag.